The flush API allows to flush one or more indices through an API. The
flush process of an index basically frees memory from the index by
flushing data to the index storage and clearing the internal
transaction log. By
default, Elasticsearch uses memory heuristics in order to automatically
trigger flush operations as required in order to clear memory.

If set to true the flush operation will block until the
flush can be executed if another flush operation is already executing.
The default is false and will cause an exception to be thrown on
the shard level if another flush operation is already running.

force

Whether a flush should be forced even if it is not necessarily needed ie.
if no changes will be committed to the index. This is useful if transaction log IDs
should be incremented even if no uncommitted changes are present.
(This setting can be considered as internal)